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Photosystems

Photosystems. Photosystem (fig 10.12) = rxn center surrounded by several light-harvesting complexes Light-harvesting complex = pigment molecules bound to proteins (act as antenna for rxn center)

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Photosystems

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  1. Photosystems Photosystem (fig 10.12) = rxn center surrounded by several light-harvesting complexes Light-harvesting complex = pigment molecules bound to proteins (act as antenna for rxn center) Rxn center = protein complex that includes 2 special chlorophyll a molecules + primary e- acceptor molecule First step of light rxns: special chlorophyll a molecule transfers its excited e- to the primary e- acceptor

  2. A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes • A photosystem • Is composed of a reaction center surrounded by a number of light-harvesting complexes Thylakoid Photosystem Photon STROMA Light-harvesting complexes Reaction center Primary election acceptor e– Thylakoid membrane Special chlorophyll a molecules Transfer of energy Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12

  3. Light-harvesting Complexes and Reaction Centers • The light-harvesting complexes consist of pigment molecules bound to particular protein • They funnel the energy from photons of light to the reaction center • When a reaction-center chlorophyll a molecule absorbs energy, one of its electrons gets bumped up to a primary electron acceptor

  4. Photosystems Two types of photosystems embedded in the thylakoid membranes of land plants (fig 10.13) 1. Photosystem I (PS I) Rxn center chlorophyll a = P700 Cyclic and noncyclic e- flow 2. Photosystem II (PS II) Rxn center chlorophyll a = P680 Noncyclic e- flow Noncyclic e- flow (fig 10.13) Uses PS II & PS I Excited e- from PS II  goes through ETC  produces ATP Excited e- from PS I  ETC  used to reduce NADP+ Electrons ultimately supplied from splitting water  releases O2 and H+ Cyclic e- flow (fig 10.15) Uses only PS I Only generates ATP Excited e- from PS I cycle back from 1st ETC No O2 release & no NADPH made

  5. Two Photosystems • The thylakoid membrane • Is populated by two types of photosystems, I and II

  6. H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH Electron Transport chain O2 [CH2O] (sugar) Primary acceptor 7 Primary acceptor 4 Fd Electron transport chain Pq 2 e 8 e– e H2O NADP+ + 2 H+ Cytochrome complex 2 H+ NADP+ reductase + 3 NADPH O2 PC e– + H+ P700 e– 5 Light P680 Light 1 6 ATP Photosystem-I (PS I) Photosystem II (PS II) Figure 10.13 Noncyclic Electron Flow – Involves both Photosystems • Produces NADPH, ATP, and oxygen, and is the primary pathway of energy transformation in the light rxns.

  7. e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon e– Photon Photosystem I Photosystem II Figure 10.14  A mechanical analogy for the light reactions

  8. Cyclic Electron Flow • Under certain conditions • Photoexcited electrons take an alternative path • Uses Photosystem I only

  9. Primary acceptor Primary acceptor Fd Fd NADP+ Pq NADP+ reductase Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II Figure 10.15 In cyclic electron flow • In cyclic electron flow • Electrons cycle back to the first ETC • Only ATP is produced

  10. Light Reactions and Chemiosmosis The light reactions and chemiosmosis: (fig 10.17) As e- are brought down their E gradient through the ETCs embedded in the thylakoid membranes  that E is being used to pump H+ into the thylakoid space ATP synthase is the only place H+ can flow along its concentration gradient  E from this flow is used to join ADP + Pi  ATP NADPH is made by NADP+ reductase ATP and NADPH  used to power Calvin cycle

  11. H2O CO2 LIGHT NADP+ ADP CALVIN CYCLE LIGHT REACTOR ATP NADPH STROMA (Low H+ concentration) O2 [CH2O] (sugar) Cytochrome complex Photosystem II Photosystem I NADP+ reductase Light 2 H+ 3 NADP+ + 2H+ Fd NADPH + H+ Pq Pc 2 H2O 1⁄2 O2 THYLAKOID SPACE (High H+ concentration) 1 2 H+ +2 H+ To Calvin cycle ATP synthase Thylakoid membrane STROMA (Low H+ concentration) ADP ATP P H+ Figure 10.17 Light reactions and chemiosmosis: organization of the thylakoid membrane As e- are brought down their E gradient through the ETCs embedded in the thylakoid membranes  that E is being used to pump H+ into the thylakoid space ATP synthase is the only place H+ can flow along its concentration gradient  E from this flow is used to join ADP + Pi  ATP

  12. Light Reactions and Chemiosmosis Comparison of chemiosmosis in mitochondria & chloroplasts (fig 10.16): ATP generation basically the same (ETC + ATP synthase) Mitochondria use high-E e- extracted from organic molecules (glucose) Chloroplasts use light E to drive e- to top of the ETC

  13. Comparison of Chemiosmosis in Chloroplasts and Mitochondria • Chloroplasts and mitochondria • Generate ATP by the same basic mechanism: chemiosmosis • But use different sources of energy to accomplish this. Chloroplasts use light energy and mitochondria use the chemical energy in organic molecules.

  14. Key Higher [H+] Lower [H+] Chloroplast Mitochondrion CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE H+ Diffusion Thylakoid space Intermembrance space Electron transport chain Membrance ATP Synthase Stroma Matrix ADP+ P ATP H+ Figure 10.16 The spatial organization of chemiosmosis • Differs in chloroplasts and mitochondria

  15. Chemiosmosis: Chloroplasts vs. Mitochondria • In both organelles • Redox reactions of electron transport chains generate a H+ gradient across a membrane • ATP synthase • Uses this proton-motive force to make ATP

  16. The Calvin Cycle • The Calvin cycle uses ATP + NADPH to convert CO2 sugar • Calvin cycle (fig 10.18) = anabolic process that builds carbohydrates from smaller molecules • Consumes E (ATP) • Uses NADPH as reducing agent for adding high E e- to make sugar • Phase 1: Carbon fixation • CO2 incorporated by attaching to 5-C sugar (ribulose bisphosphate = RuBP) • Catalyzed by Rubisco (ribulose bisphosphate carboxylase/oxygenase) = most abundant protein on Earth! • Unstable 6-C intermediate  2 molecules of 3-phosphoglycerate (3-C sugar) • Phase 2: Reduction • 3-phosphoglycerate has phosphate added  NADPH reduces intermediate  G3P (glyceraldehyde-3-phosphate) • One G3P exits the cycle to be used by the plant cell • Other 5 recycled to regenerate RuBP • Phase 3: Regeneration of CO2 acceptor (RuBP) • Requires ATP • Five G3P (3-C)  Three 5-C molecules of RuBP

  17. Calvin cycle uses ATP & NADPH to convert CO2 to sugar • The Calvin cycle • Is similar to the citric acid cycle • Occurs in the stroma

  18. The Calvin Cycle Calvin cycle (fig 10.18) = anabolic process that builds carbohydrates from smaller molecules (CO2 and H2O Consumes E (ATP) from the light reactions Uses NADPH (from the light reactions) as the reducing agent for adding high energy e- to make sugar

  19. H2O Input CO2 Light 3 (Entering one at a time) NADP+ CO2 ADP CALVINCYCLE LIGHTREACTION ATP NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-livedintermediate P 6 3 P P Ribulose bisphosphate(RuBP) 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 ADP 6 P P 3 ATP 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ 6 P P 5 (G3P) 6 P Glyceraldehyde-3-phosphate (G3P) P 1 Glucose andother organiccompounds G3P(a sugar)Output Figure 10.18 The Calvin cycle • The Calvin cycle Phase 1: Carbon fixation Phase 3:Regeneration ofthe CO2 acceptor(RuBP) Phase 2:Reduction

  20. The Calvin cycle has three phases • The Calvin cycle has three phases • Carbon fixation • Reduction • Regeneration of the CO2 acceptor

  21. H2O Input CO2 Light 3 (Entering one at a time) NADP+ CO2 ADP CALVINCYCLE LIGHTREACTION ATP NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-livedintermediate P 6 3 P P Ribulose bisphosphate(RuBP) 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 ADP 6 P P 3 ATP 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ 6 P P 5 (G3P) 6 P Glyceraldehyde-3-phosphate (G3P) P 1 Glucose andother organiccompounds G3P(a sugar)Output The Calvin cycle • Phase 1: Carbon fixation • CO2 incorporated by attaching to 5-C sugar (ribulose bisphosphate = RuBP) • Catalyzed by Rubisco (ribulose bisphosphate carboxylase/oxygenase) = most abundant protein on Earth! • Unstable 6-C intermediate  2 molecules of 3-phosphoglycerate (3-C sugar) Phase 1: Carbon fixation Phase 3:Regeneration ofthe CO2 acceptor(RuBP) Phase 2:Reduction

  22. H2O Input CO2 Light 3 (Entering one at a time) NADP+ CO2 ADP CALVINCYCLE LIGHTREACTION ATP NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-livedintermediate P 6 3 P P Ribulose bisphosphate(RuBP) 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 ADP 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH 6 NADPH+ 6 P P 5 (G3P) 6 P Glyceraldehyde-3-phosphate (G3P) P 1 Glucose andother organiccompounds G3P(a sugar)Output The Calvin cycle • Phase 2: Reduction • 3-phosphoglycerate has phosphate added  NADPH reduces intermediate  G3P (glyceraldehyde-3-phosphate) • One G3P exits the cycle to be used by the plant cell • Other 5 recycled to regenerate RuBP Phase 1: Carbon fixation Phase 3:Regeneration ofthe CO2 acceptor(RuBP) Phase 2:Reduction

  23. H2O Input CO2 Light 3 (Entering one at a time) NADP+ CO2 ADP CALVINCYCLE LIGHTREACTION ATP NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-livedintermediate P 6 3 P P Ribulose bisphosphate(RuBP) 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 ADP 6 P P 3 ATP 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ 6 P P 5 (G3P) 6 P Glyceraldehyde-3-phosphate (G3P) P 1 Glucose andother organiccompounds G3P(a sugar)Output The Calvin cycle • Phase 3: Regeneration of CO2 acceptor (RuBP) • Requires ATP • Five G3P (3-C)  Three 5-C molecules of RuBP Phase 1: Carbon fixation Phase 3:Regeneration ofthe CO2 acceptor(RuBP) Phase 2:Reduction

  24. Balancing gas exchange against water loss Terrestrial plants have to balance gas exchange (CO2 + O2) w/ H2O loss Happens at the stomata on leaves If a plant closes its stomata during hottest part of day  accumulation of O2 & no CO2 uptake Photorespiration = process that uses O2 instead of CO2 only generates 1/2 the amount of 3-phosphoglycerate decreases Ps output by siphoning organic material from Calvin cycle (up to 50%) Rubisco can act on both CO2 & O2 (not discriminate) C3 plants Most plants Use only Calvin cycle to fix CO2 in mesophyll Limited by photorespiration

  25. On hot, dry days, plants close their stomata • Conserving water but limiting access to CO2 • Causing oxygen to build up

  26. Photorespiration: An Evolutionary Relic? • In photorespiration • O2 substitutes for CO2 in the active site of the enzyme rubisco • The process consumes oxygen and releases CO2 • The photosynthetic rate is reduced

  27. Adaptations to Hot, Arid Climates • Alternative mechanisms of carbon fixation have evolved in hot, arid climates • C4 plants separate initial carbon fixation from the Calvin cycle in space • CAM plants separate initial carbon fixation from the Calvin cycle in time

  28. Adaptations to Hot, Arid Climates • C4 plants (fig 10.21a) - exhibit a spatial separation of C-fixation • Preface Calvin cycle with alternate mode of C-fixation that forms a 4-C compound (occurs in mesophyll cells) – Ex. Corn and sugarcane • Associated w/ unique leaf anatomy (Kranz anatomy, fig 10.19) • PEP carboxylase adds CO2 to PEP (phosphoenolpyruvate)  4-C oxaloacetate  converted to malate  transported to bundle sheath cells broken down to CO2 (Calvin cycle) + pyruvate (3-C goes back to mesophyll to regenerate PEP) • PEP carboxylase has a much higher affinity for CO2 than rubisco does, and no affinity for O2 • CAM plants (fig 10.21b) Ex. Succulents, cacti, pineapples, etc. • Crassulacean Acid Metabolism plants adapted to arid environments • Temporal separation of C-fixation: open stomata at night and close during day • Take up CO2 at night  oxaloacetate  malate  malic acid stored in vacuole during the day • ATP + NADPH synthesized during day  malic acid transported out of vacuole  malate  CO2 + pyruvate at night

  29. Adaptations to Hot, Arid Climates • C4 plants (fig 10.21a) • CAM plants (fig 10.21b) • These types of plants separate initial carbon fixation from the Calvin Cycle either in space or time. • This allows them to keep their stomata partially closed during the day to minimize water loss.

  30. C4 Plants • C4 plants minimize the cost of photorespiration • By incorporating CO2 into four carbon compounds in mesophyll cells • These four carbon compounds • Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle

  31. Mesophyll cell Mesophyll cell Photosynthetic cells of C4 plant leaf PEP carboxylase Bundle- sheath cell CO2 CO2 PEP (3 C) Oxaloacetate (4 C) ADP Vein (vascular tissue) Malate (4 C) ATP C4 leaf anatomy Pyruate (3 C) Bundle- Sheath cell CO2 Stoma CALVIN CYCLE Sugar Vascular tissue Figure 10.19 C4 leaf anatomy and the C4 pathway • Separates initial carbon fixation from the Calvin cycle in space

  32. CAM Plants • CAM plants • Open their stomata at night, incorporating CO2 into organic acids • During the day, the stomata close • And the CO2 is released from the organic acids for use in the Calvin cycle • So they separate initial carbon fixation from the Calvin cycle in time.

  33. 2 1 Pineapple Sugarcane C4 CAM CO2 CO2 Mesophyll Cell Night CO2 incorporated into four-carbon organic acids (carbon fixation) Organic acid Organic acid Bundle- sheath cell (b) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times. Day (a) Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. Organic acids release CO2 to Calvin cycle CALVINCYCLE CALVINCYCLE Sugar Sugar Figure 10.20 CAM pathway is similar to the C4 pathway

  34. Importance of Photosynthesis • About 50% of the organic material made by Ps is consumed as fuel for respiration in the plant body • Not all plant cells make their own food  have to be supplied by Ps cells • Two most important products we derive from plants come directly from Ps (what are they?) • Fig 10.21 = nice overview of Ps

  35. Light reaction Calvin cycle H2O CO2 Light NADP+ ADP + P1 RuBP 3-Phosphoglycerate Photosystem II Electron transport chain Photosystem I ATP G3P Starch (storage) NADPH Amino acids Fatty acids Chloroplast O2 Sucrose (export) Calvin cycle reactions: • Take place in the stroma • Use ATP and NADPH to convert CO2 to the sugar G3P • Return ADP, inorganic phosphate, and NADP+ to the light reactions Light reactions: • Are carried out by molecules in the thylakoid membranes • Convert light energy to the chemical energy of ATP and NADPH • Split H2O and release O2 to the atmosphere Figure 10.21 The Importance of Photosynthesis: A Review • A review of photosynthesis

  36. Two most important products of photosynthesis • Organic compounds produced by photosynthesis • Provide the energy and building material for ecosystems • Oxygen produced by photosynthesis • provides an aerobic environment that allows for cellular respiration

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